1. Field of the Invention
The present invention relates to a throttle body configured to control an amount of air flowing toward an air-intake port of an engine. More particularly, the present invention relates to a tandem valve type throttle body including a main throttle valve that is mounted within an air-intake passage formed in the throttle body and that is configured to be controlled to open and close by an accelerator lever, and a sub-throttle valve that is mounted within a region of the air-intake passage which is located upstream of the main throttle valve in an air flow and that is configured to be operated to open and close by an actuator such as an electromagnetic actuator, and an engine of a motorcycle having the throttle body.
2. Description of the Related Art
A conventional tandem valve type throttle body is disclosed in Japanese Patent Application Publication No. 2003-83171, owned by one assignee of the subject application Keihin Corporation. Turning to FIGS. 3 and 4, a conventional tandem valve type throttle body is shown generally. A throttle body T is constructed to contain an air-intake passage 10 extending therein laterally relative to an engine. A main throttle valve member 12 is located within a downstream air-intake passage 10b of the throttle body T. As used herein, “upstream” and “downstream” are meant to define a direction of a flow of air taken in from outside. The main throttle valve member 12 is attached to a main throttle valve shaft 11 rotatably mounted to the throttle body T. The main throttle valve shaft 11 is rotatably operated through a mechanical or electric system by an operator (rider). The main throttle valve member 12 is configured to substantially open and close the air-intake passage 10 according to the rotation of the main throttle valve shaft 11.
A sub-throttle valve member 13 is mounted within an upstream air-intake passage 10a located upstream of the main throttle valve member 12. The sub-throttle valve member 13 is attached to a sub-throttle valve shaft 14 rotatably mounted to the throttle body T. The sub-throttle valve shaft 14 is configured to be operated to open and close by an electromagnetic actuator such as a motor. The sub-throttle valve member 13 is forcibly closed, for example, during traction control. The downstream air-intake passage 10b within which the main throttle valve member 12 is mounted has a cross-section of a perfect circle shape with a diameter “d”. The upstream air-intake passage 10a within which the sub-throttle valve member 13 is mounted has a cross-section of a perfect circle shape with a diameter “D”. The diameter “D” is larger than the diameter “d” (D>d). Such a construction is capable of reducing air-intake resistance of air flowing through the downstream air-intake passage 10b. 
When applied to a multi-cylinder engine, a plurality of throttle bodies T are arranged to extend laterally relative to an engine block of the engine. In the case of a three-cylinder engine, as shown in FIG. 4, three throttle bodies T are arranged to extend laterally relative to the engine block in the following order from the left to the right: a first throttle body T1, a second throttle body T2, and a third throttle body T3.
When the plurality of throttle bodies T are arranged to extend laterally relative to the engine block of the multi-cylinder engine, the sub-throttle shafts 14 are formed by a common shaft, which extends transversely through diameters of the air-intake passages 10a of the throttle bodies T1, T2, and T3. The common shaft is rotatably supported by bearing holes (left and right bearing holes) 15a and 15b on left and right sides of the corresponding air-intake passage 10a. 
As shown in FIG. 4, a right end of the common shaft protrudes rightward from the third throttle body T3 and is connected to a motor M which is configured to be controlled and driven by an ECU (engine control unit) via a gear mechanism (gear train). Air-intake pipes are respectively connected to the downstream air-intake passages 10b of the throttle bodies T1, T2, and T3 and are connected to cylinders of the engine, although the air-intake pipes and the engine are not shown in FIG. 4. The upstream air-intake passages 10a of the throttle bodies T1, T2, and T3 are respectively connected to an air cleaner box 16 (see FIG. 3). A filter element (air filter) such as a sponge (not shown) is accommodated within the air cleaner box 16. The filter element is capable of filtering air taken in from outside to remove unwanted substances therefrom, and the resulting clean air is supplied to the upstream air-intake passages 10a of the throttle bodies T1, T2, and T3.
In order to allow the air to flow efficiently from the downstream air-intake passage 10b toward the cylinders of the engine in the tandem valve type throttle body provided with the conventional sub-throttle valve member 13, a cross-sectional area of the upstream air-intake passage 10a located upstream of the sub-throttle valve member 13 may be increased. In this case, the cross-sectional area of the upstream air-intake passage 10a having a cross-section with a perfectly circular shape may be increased by increasing a diameter of the upstream air-intake passage 10a from “D” to “D1” (D1>D). As shown in FIG. 4, the upstream air-intake passage 10a with the increased diameter “D1” is represented by a dotted line.
In accordance with the construction of the air-intake passage 10a with the increased diameter “D1”, the amount of the air flowing from the upstream air-intake passage 10a toward the downstream air-intake passage 10b increases, and hence efficiency of the air flow from the downstream air-intake passage 10b toward the cylinders of the engine increases. However, the following problems may arise.
First, a dimension G of an outer shape of the entire throttle bodies T1, T2, and T3 in a width direction of a motorcycle increases from G1 to G2 (G2>G1). If the dimension G of the outer shape of the throttle bodies T1, T2, and T3 increases, then the conventional air cleaner box 16 cannot be employed. So, there is a need for an air cleaner box having a larger size or another configuration. Such an air cleaner box is difficult to mount in a limited storage space of the motorcycle. In addition, if the sizes of the air cleaner box and the throttle bodies increase in the width direction of the motorcycle, then the motorcycle becomes larger, which may force the rider to open legs when straddling the motorcycle. That is, the rider cannot ride the motorcycle in a natural posture.
Second, since the bearing holes of the throttle body must be varied, a length of bearing portions of the sub-throttle valve shaft decreases. For example, the length of the bearing portion corresponding to the first bearing hole 15a decreases from “L1” to “L2” (L1>L2), and the length of the bearing portion corresponding to the second bearing hole 15b decreases from “L3” to “L4”.
If the length of the bearing portion of the sub-throttle valve shaft thus decreases, it becomes necessary to re-conduct a durability test for the bearing portion. This significantly increases the number of steps for a check test of a material and treatment (heat treatment or surface treatment, etc) of the sub-throttle valve shaft 14.
As a solution to the first problem, pitches P1 and P2 of adjacent throttle bodies T1, T2, and T3 may be reduced to reduce the dimension G. As a solution to the second problem, the pitches P1 and P2 may be increased to increase the length “L” of the bearing holes 15a and 15b. However, the pitches P1 and P2 are determined according to the arrangement of the air-intake passages (intake ports) of the engine, and hence are incapable of being easily changed. The change in the arrangement of the air-intake passages unavoidably results in significant design change in the engine, and is extremely difficult.
Japanese Patent Application No. 2004-87118, owned by another assignee of the subject application, KAWASAKI JUKOGYO KABUSHIKI KAISHA, discloses an air-intake pipe having a passage with a cross-section having long and short axes and a throttle valve pivotally mounted within the passage of the air-intake pipe.